The present invention relates, in general, to hydrostatic transmission assemblies, and more particularly to a method and apparatus for increasing and extending the torque and speed range of a hydrostatic transmission.
Hydrodynamic transmissions have a relatively wide range of speed and torque ratios, and they are, accordingly, readily adapted for use in automobiles and trucks. Hydrostatic transmissions or drives, by contrast, have heretofore had a limited range of speed or torque output, and have accordingly been more commonly used in special purpose vehicles, such as tracked vehicles. If the range of the torque and/or speed ratio of a hydrostatic drive or transmission could be increased, there are numerous additional applications to which such drives could be put with attendant substantial advantage. Hydrostatic drives, for example, provide infinite control of output speed, regardless of engine speed. Thus, a vehicle requiring power to run auxillary equipment mounted on the vehicle and to drive the vehicle wheels simultaneously can be driven with one engine.
Prior art hydrostatic transmission assemblies have included manually shiftable transmissions which can be used to change the gear reduction ratio between the hydraulic motor of the hydrostatic portion of the transmission and the drive wheels or load. Such transmission assemblies, however, have required that the vehicle be stopped in order to shift to a new drive range. Another prior art approach has been to employ a hydrostatic transmission and a power shift apparatus similar to an automatic transmission. Such transmission assemblies have not included any means for sychronous shifting, with the result that the system experiences serious hydraulic pressure surges and momentary torque peaks.
A hydrostatic transmission assembly is disclosed in U.S. Pat. No. 4,192,199 in which a mechanical transmission is coupled to a hydrostatic transmission and an attempt has been made to address the problem of hydraulic pressure surges and torque peaks. The hydrostatic transmission employs a variable displacement motor. Upon shifting of the mechanical transmission, feedback from the load is isolated from the pump control circuit, and the pump is stroked or destroked during the shift. While attempting to control the pump during shifting of the mechanical transmission can eliminate hydraulic pressure surge problems, the overall transmission assembly still has a relatively limited speed and torque range. Moreover, the hydraulic pump, which has an infinite torque ratio, must be controlled by a relatively sophisticated control circuit to attempt to minimize surges, and the system is not inherently synchronized so that failures in the control circuit will produce surges and peaks.
Other attempts have been made to combine the hydrostatic drives with mechanical gear assemblies or transmissions. U.S. Pat. Nos. 3,620,130 and 3,300,000 also are illustrative of hydrostatic drives which included variable displacement pumps and variable displacement motors that have been combined with mechanical transmissions having involved planetary gear trains. In U.S. Pat. No. 3,834,164, a controller having variable volume has been added to the hydrostatic drive. Another approach shown in the prior art is to employ a variable displacement pump with a fixed displacement motor and provide either a specialized control mechanism or a mechanical gear assembly coupled to the motor to achieve various goals. U.S. Pat. Nos. 4,363,247, 4,005,577, 3,302,478 and 2,543,584 are typical of such approaches. In U.S. Pat. No. 4,319,500 a gear train is interposed between the hydraulic pump and the motor, while in U.S. Pat. No. 3,612,202 multiple axles are employed to overcome the relatively limited torque range inherent in the hydrotatic drive system. Finally, U.S. Pat. No. 3,338,115 illustrates a hydrokinetic drive.
While the above set forth prior art hydrostatic drive assemblies or systems have enabled the attainment of various advantages, they still do not afford a simple yet effective apparatus or method for the extension of the relatively limited torque and speed range inherent in a hydrostatic transmission. To the extent such systems do extend the torque or speed range, the improvement in range tends to be relatively limited. Alternatively, the system becomes extremely complex and often directed toward performance characteristics which will not make the drive system suitable for applications in which a high range of torque and speed is required.